This invention relates to the processing of wastewater, and particularly to a method for providing treatment to achieve Class A biosolids.
Land application of treated wastewater sludge (biosolids) is controversial because the biosolids therein potentially contain human pathogens, i.e., some counties in California have banned the application of biosolids which meet the Environmental Protection Agency""s (EPA) Class B pathogen reduction standards, as set forth in 40 CFR 503, which require a fecal coliform level of less than two million most-probable-number (MPN) per gram of total solids, but not Class A pathogens standards per 40 CFR 503. Wastewater treatment agency managers are concerned about the future of biosolids land applications in their states. EPA""s Class A pathogen standards require the virtual elimination of pathogens in biosolids. In accordance with 40 CFR 503.32 (a)(5), Class Axe2x80x94alternate 3 and 40 CFR 503.32(a)(6)xe2x80x94alternate 4, EPA Class A pathogen requirements are met in biosolids when fecal coliform densities are less than 1,000 MPN per gram total solids; or when Salmonella densities are less than 3 MPN per four grams total solids. Additionally, enteric virus must be less than 1 plaque-forming unit per four grams of total solids, and helminth ova is less than one viable helminth ova per four grams of total solids. Meeting Class A standards will significantly increase the opportunity for biosolids recycling, however, known processes which achieve Class A pathogen levels in biosolids are generally cost prohibitive. The provision of a low-cost method of biosolids treatment, which will meet Class A standards will offer additional biosolids management options Known thermophilic anaerobic digestion processes are not classified as a Process to Further Reduce Pathogens (PFRP) under 40 CFR Part 503.
Anaerobic digestion has been one of the most widely used processes for the stabilization of primary and secondary sludges produced at municipal wastewater treatment facilities. The majority of applications of anaerobic digestion to wastewater sludges have been in the mesophilic temperature range, from 35xc2x0 C. to 40xc2x0 C. (95xc2x0 F. to 104xc2x0 F.). Anaerobic sludge digestion in the thermophilic temperature range from 45xc2x0 C. to 65 xc2x0 C. (113xc2x0 F. to 149xc2x0 F.) has been practiced to only a limited extent.
The limited use of anaerobic digestion at temperatures above the mesophilic range is due to higher energy requirements to obtain the higher thermophilic temperature, poor process stability, increased odor, and lower quality supernatant (filtrate/centrate). The advantages of thermophilic anaerobic digestion over mesophilic anaerobic digestion have accrued from increased stabilization and methane production rates, and from improvements in sludge dewatering properties. Since the advent of the 40 CFR Part 503 Regulations, more studies have focused on the destruction of pathogenic organisms.
Thermophilic anaerobic digestion has an advantage of improving pathogen destruction, and has the potential to meet the pathogen quality requirements of EPA""s Class A biosolids. While the economic disadvantages of thermophilic anaerobic digestion has outweighed the advantages of the process, the implementation of 40 CFR Part 503 and the use of a two-stage digestion system, having a thermophilic or mesophilic first-stage and a mesophilic or thermophilic second-stage, may negate the economic disadvantage.
Recent studies have introduced some improvement to the stability and performance of the thermophilic anaerobic digestion process. These improvements include:
Fast start-up method. The fast start-up method produces a more robust thermophilic culture that is relatively tolerant of temperature fluctuations and increases process stability.
Draw/fill operation. The draw/fill cycle improves pathogen removal by reducing the potential for short-circuiting.
Multiple-stage digestion. Multiple-stage reactors reduce the potential for short-circuiting, wherein flow exits the reactor before it should, provide a low pH environment in the first stage if methanogenesis is minimized or eliminated in this stage, which apparently enhances pathogen reduction; and, if the second-stage reactor is mesophilic, produce a higher quality sludge in terms of odor and supernatant.
Research has shown that pathogen destruction in municipal sludge digestion follows a typical time/temperature relationship, wherein higher temperatures require shorter exposure times for pathogen destruction. Data has been collected demonstrating survival rates of various pathogens in municipal sludge digestion. These data suggest that thermophilic digestion achieves pathogenic bacteria reduction rates about two orders of magnitude higher than mesophilic digestion, and may meet the pathogenic bacteria levels required for Class A sludge. EPA studies and full-scale research results indicate enhanced pathogen destruction in acid-phase digestion.
U.S. Pat. No. 5,525,228 to Dague et al., for Temperature-phased anaerobic waste treatment process, granted Jun. 11, 1996, describes a process having a long term hydraulic retention time (HRT) and which operates in a temperature range sufficient to Pasteurize the waste.
U.S. Pat. No. 5,624,565 to Lefevre et al., for Method of regulating aeration in a basin for biological treatment of Waste water, granted Apr. 29, 1997, describes a method which requires elimination of carbon in aerobiosis, nitrification and denitrification.
U.S. Pat. No. 5,746,919 to Dague et al., for Temperature-phased anaerobic waste treatment process, granted May 5, 1998, is a continuation of the ""228 patent.
U.S. Pat. No. 5,900,150 to Fayoux et al., for Method of purifying waste water biologically, granted May 4, 1999, describes a treatment method which is conducted at temperatures between 25xc2x0 C. and 40xc2x0 C.
A method of treating a waste stream comprising feeding a waste stream into a reactor constructed and arranged for a fast start and maintained in a thermophilic temperature regime of between about 50xc2x0 C. and 62xc2x0 C., for a HRT of less than or equal to 48 hours, wherein the reactor contents are maintained at a pH of 6.5 or less; drawing a portion of the reactor contents and feeding into a second reactor which is maintained in a mesophilic temperature regime of between about 28xc2x0 C. to 38xc2x0 C. for a HRT of at least thirteen days; and replacing the volume of sludge drawn from the thermophilic reactor with more waste from the waste stream. Alternately, the waste stream may be initially treated in a mesophilic reactor, followed by treatment in a thermophilic reactor, or a single stage thermophilic reactor may be used.
An object of the invention is to meet Class A requirements with a thermophilic anaerobic digestion processes.
Another object of the invention is to provide a digestion process for fecal coliform, Salmonella, enteric virus and helminth ova reduction.
A further object of the invention is to provide a treatment method which will produce less methane than do prior art methods.
Another object of the invention is to provide a treatment method which will produce biosolids which are substantially pathogen-free and are suitable for application with respect to pathogen densities where ever commercial fertilizers or soil conditioners are used.
This summary and objectives of the invention are provided to enable quick comprehension of the nature of the invention. A more thorough understanding of the invention may be obtained by reference to the following detailed description of the preferred embodiment of the invention in connection with the drawings.